The present invention relates to a surface defect test apparatus for inspecting a defect on a surface of a rod-shaped object, although not restrictive, such as a mechanical part e.g. bearing roller which is casted, sintered or machined or an uranium pellet for nuclear fuel, said defect particularly influencing to performance and property of the object.
An example of prior art surface defect test apparatus is shown in FIG. 1, which is a defect test apparatus utilizing a photo-electric conversion element such as a photo-transistor. (The article "Automatic Inspection of Surface Defect of Function Component" disclosed in pages 117-124 of Japanese Journal "Mechanical Automation" Vol. 5 No. 12 1973). In FIG. 1, light emitted from a light source 1 is focused by a lens 3 and directed onto a surface 5 of an object, in a spot, from an oblique direction a reflecting light therefrom is focused on a photo-electric conversion element 2 through a lens 4 in a normal reflection direction, or a diffused reflection light is focused on a photo-electric conversion element 2' through a lens 4' in a diffused reflection direction, depending on the surface condition of the illuminated portion of the object. A detection head comprising the light source 1, the photo-electric conversion element 2 or 2', the lens 3 and the lens 4 or 4' is moved to scan the surface 5 of the object to test the surface 5.
FIG. 2a to FIG. 2g show test results. FIG. 2a shows a sectional view of the surface 5 of the object, in which, by way of example, numeral 6 denotes a large defect and numeral 7 denotes a small defect. It is assumed that the light spot is scanned in the direction of an arrow. When the defects 6 and 7 exist on the surface 5 of the object, the reflected light therefrom is scattered so that the amount of reflected light directed to the photo-electric conversion element 2 arranged in the direction of normal reflection decreases and the light directed to the lens 3 is not focused into a complete spot. Accordingly, depending on the size of defect, an output level of a detected signal 8 with respect to time t derived from the photo-electric conversion element 2 changes as shown in FIG. 2b. When the detected signal 8 is discriminated by two threshold levels 9 and 10, di-value signals 11 and 12 shown in FIG. 2c and FIG. 2d are produced. When the diffused reflection light is received by the photo-electric conversion element 2' arranged directly above the illuminated portion, a detected signal 8' as shown in FIG. 2e is produced, which signal is inverse to the detected signal shown in FIG. 2b in their output levels (brightness level). When the detected signal 8' is discriminated by two threshold levels 9' and 10', di-value signals 11' and 12' as shown in FIG. 2f and FIG. 2g are produced.
Thus, the signals 11 and 11' which are discriminated by higher threshold levels 9 and 9' each has two peaks corresponding to the large defect 6 and the small defect 7 while the signals 12 and 12' which are discriminated by the lower threshold levels 10 and 10' each has one peak corresponding to the large defect 6. Accordingly, by comparing the signals 11 and 11' with the signals 12 and 12', the size of the defect can be determined.
However, in the prior art appatatus of this type in which the defect is detected by sensing the normal reflection light or diffused reflection light from the surface 5 of the object by the photo-electric conversion element 2 or 2', it is possible to determine the size of the defect on the surface of the object but it is difficult to classify and evaluate various defects such as broken cavity, crack and pinhole (pit) by sensing the normal reflection light and the diffused reflection light.
As an alternative, it is known to inspect the defect of the object by an air gauge. (The article "Automatic Inspection of Fuel element Pellets" disclosed in "Metology and Inspection" Autumn 1969 published by Vernon Instrument Co. Ltd.). Since this system has a low resolution power, it can detect and evaluate only large defects but it cannot classify and evaluate various small defects such as broken cavity, crack and pinhole (pit). It also requires longer test time.